1. Field of the Invention
The invention relates to reducing the benzene content of reformate by alkylation and/or transalkylation.
2. Description of Related Art
The present invention relates to an unusual way of upgrading some of the lower value products of two mature processes, catalytic reforming and those producing aromatic rich heavy streams as low value products or by-products, e.g., cycle oils from a catalytic cracking process.
Catalytic reforming of naphtha boiling range feeds over platinum based catalyst to produce high octane reformate has been one of the most successful processes in the world. More than a hundred units are in use, converting low octane naphthas to high octane, aromatic rich gasoline. The only problem with the process is that the product inherently contains large amounts of aromatics, including benzene. Many localities are limiting the amount of benzene which can be contained in gasoline, because of the toxic nature of benzene. Another minor problem in some catalytic reforming units is that the octane number of the gasoline produced varies significantly with boiling range. The light reformate, e.g, the C6- fraction, sometimes has a lower octane than desired and lower than the octane of the C7+ fraction. The C6- fraction can be doubly troubling to refiners, having a shortage of octane and an excess of benzene.
Many processes produce relatively heavy, aromatic rich by-product streams. These are generally characterized by the presence of relatively large amounts of fused polycyclic aromatic compounds which are relatively refractory to further processing, and are generally of low value. FCC cycle oils, coker gas oils, and aromatic extracts from lubricant manufacturing facilities are typical of such streams. Cycle oils from catalytic cracking are the most widely available, so the catalytic cracking process will be briefly reviewed.
Catalytic cracking of hydrocarbons has enjoyed worldwide success. It is probably the method of choice for converting a heavy feed into lighter, more valuable products. Catalytic cracking of hydrocarbons is carried out in the absence of externally supplied H2, in contrast to hydrocracking, in which H2 is added during the cracking step. An inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In the fluidized catalytic cracking (FCC) process, hydrocarbon feed contacts catalyst in a reactor at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The hydrocarbons crack, and deposit carbonaceous hydrocarbons or coke on the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, and is then regenerated. In the catalyst regenerator, the coke is burned from the catalyst with oxygen containing gas, usually air. Coke burns off, restoring catalyst activity and simultaneously heating the catalyst to, e.g., 500.degree. C-900.degree. C., usually 600.degree. C.-750.degree. C. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Older FCC units regenerate the spent catalyst in a single dense phase fluidized bed of catalyst. Although there are myriad individual variations, typical designs are shown in U.S. Pat. No. 3,849,291 (Owen) and U.S. Pat. No. 3,894,934 (Owen et al), and U.S. Pat. No. 4,368,114 (Chester et at.) which are incorporated herein by reference.
Most new units are of the High Efficiency Regenerator (H.E.R.) design using a coke combustor, a dilute phase transport riser, and a second dense bed, with recycle of some hot, regenerated catalyst from the second dense bed to the coke combustor. Units of this type are shown in U.S. Pat. No. 3,926,778 (which is incorporated by reference) and many other recent patents. The H.E.R. design is used in most new units.
Another type of catalytic cracking process is moving bed catalytic cracking, or Thermofor Catalytic Cracking (TCC), which is the moving bed analogue of the FCC process.
Both FCC and TCC produce a spectrum of cracked products, ranging from light ends, through heavier products including light and heavy cycle oils. The cycle oils are relatively aromatic streams, rich in single and fused ring alkyl aromatics, i.e., one or perhaps more aromatic rings having single or multiple alkyl side chains attached. These streams are produced in abundance in every cat cracker. They are difficult to upgrade by recycling to the cat cracker in large part because of the large percentage of fused ring aromatic species present. Heavy cycle oil, when recycled to the FCC, usually makes dry gas and coke, with very little gasoline boiling range product produced. The fused ring alkyl aromatics are very stable, and rather than crack to lighter liquid products they tend to dealkylate to form low value light ends, with the dealkylated fused rings condensing to form coke.
The above discussion merely reviews two mature technologies which are widely used, and which produce relatively low value streams, C6 reformate and cycle oils.
We wanted a way to overcome the problem of too much benzene in reformate, at reasonable cost. We at first eliminated the obvious ways of converting the benzene, e.g., use of aromatics extraction units to get a pure (benzene and heavy [light] cycle oil) weight alkylation of the purified benzene with a light olefin. This is a popular way to make toluene, ethylbenzene, and xylene, but the cost of purification and expense of alkylation can not be justified for producing gasoline with a low benzene content.
Others have worked on solving the same problem, such as the work reported in U.S. Pat. No. 4,209,383 (Herout et al). This patent addressed some of the problems of cost containment while converting the benzene. A low benzene content gasoline was made, at reasonable cost, by combining a catalytic reformate and a stripped liquid produced in the gas concentration unit of an FCC. The combined stream was fractionated in a dehexanizer to produce a stream rich in benzene and C3-C4 olefins. This stream was passed to an alkylation zone, where the benzene reacted with the olefins. Fractionation, rather than solvent extraction, was used to achieve some concentration of the benzene fraction. Some capital and operating cost reductions were achieved by mixing the reformate, and the light liquid from the gas con, and fractionating both in the same fractionator. The light ends from the dehexanizer were passed to an alkylation zone, one preferably using solid phosphoric acid catalyst. Although this approach would surely work to reduce the benzene content of a reformate, it does so by consuming light olefins, which many refiners would prefer to convert to non-aromatic gasoline by HF or sulfuric acid alkylation.
Owen, in U.S. Pat. No. 3,969,426, which is incorporated herein by reference, reported that a mixture of durene, benzene and toluene could be converted in a bench scale riser reactor to a substantially durene-free, high quality gasoline product with only a trace loss of carbon to gas or coke. The feed consisted of a mixture of durene (20 wt%) benzene (20 wt%) and toluene (60 wt%). The riser reactor used clean burned, 15 wt% REY zeolite catalyst having a 67.5 FAI. The riser reactor inlet mix temperature was about 800.degree. F., and the cat:oil ratio was 10.12. Essentially complete aromatic carbon retention was achieved, with less than 1 wt% of the feed going to coke, and about 0.5 wt% going to gas. Durene levels were reduced from 20 wt% to 0.2-0.4 wt%. Benzene levels were reduced from 20.0 wt% (feed) to 16.64 to 16.95 wt% (gasoline product).
This reduced the benzene content, but required the addition of durene. The durene, if not almost completely consumed, could appear in the gasoline product and cause problems because of durene's high melting point. The durene tends to remain in the gasoline boiling range product, so if poor conversion of durene occurs the gasoline product may require extensive reprocessing to reduce the durene content to acceptable levels. This approach also requires a source of durene, which is readily available only from methanol to gasoline plants.
We also investigated hydrocracking. Some limited experimental work has been reported on hydrocracking of cycle oils from FCC units. Hydrocracking will be briefly reviewed, and then the experiments, which indicated that cycle oils were better at producing benzene than removing it.
Hydrocracking, like catalytic cracking, is a way to changing the boiling range of a heavy hydrocarbon product. High hydrogen partial pressures, and high or moderate pressures are usually used to convert heavy hydrocarbons into lighter hydrocarbons. Fairly severe hydroprocessing of refractory cycle oils, to saturate them and make them susceptible to cracking in an FCC unit is well known but is not reviewed here.
Hydrocracking FCC Light Cycle Oil and Tetralin Mixtures, in U.S. Pat. No. 4,02,323 Chen et al, occurred at moderate pressure. The tetralin was reported to undergo isomerization, ring opening, dealkylation, alkylation and disproportionation reactions to yield products boiling above and below tetralin. The C5-400.degree. F. fractions consisted mainly of BTX, with a ratio of 2:1:1 (benzene:toluene:xylene).
Both tetralin and FCC cycle oils are known as hydrogen donors. Based on Chen's work, we would have expected a net production of benzene from any fairly severe processing of such hydrogen donor streams.
We then ran some experiments, and found that by selecting the proper operating conditions, and catalyst, and by using a special cofeed, we could achieve the opposite effect, i.e., convert benzene, rather than produce it.
We discovered a way to reduce the benzene content of reformate by reacting it with relatively low value, fused ring alkyl aromatic streams such as cycle oils derived from catalytic cracking units. In contrast to Chen's work, wherein tetralin, and perhaps light cycle oil, was converted to benzene, we were able to react benzene with light cycle oil and reduce the benzene content of the reformate.
Neither catalytic cracking nor hydrocracking are considered reversible reactions, i.e., both processes convert heavier feeds to lighter materials. Neither process is used for the reverse reaction, i.e., to make heavy hydrocarbons from lighter hydrocarbons.
We do not know the exact reaction mechanism by which benzene is converted, but we believe that a significant amount of alkylation and/or transalkylation occurs. We know the best benzene cofeeds are those which contain relatively large numbers of alkyl polynucleararomatics with multiple alkyl side chains. It was surprising that cycle oils, which are a complex mixture of myriad hydrocarbon species, could be used to efficiently convert benzene in reformate to something else. The use of fused ring alkyl aromatics, in preference to alkyl aromatics, permits selection of reaction conditions which promote alkylation or transalkylation reactions with benzene in reformate, without forming more benzene by dealkylation.